Solve for $n$ : $4 = 13 + n$
Answer: Subtract $13$ from both sides: $ \begin{eqnarray} \\ \hphantom{4 {- 13}} &\hphantom{=}& \hphantom{n} \\ 4 &=& 13 + n \\ \\ {-13} && {-13} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 4 {- 13} &=& n \\ \end{eqnarray} $ Simplify. $ n = -9$